This invention relates to inclusion pigments based on zirconium silicate as a casing component and cobalt-free spinels as discrete included colored compounds. The invention also relates to a process for the preparation of the inclusion pigments and the use thereof.
Colored substances of the spinel type such as CoAl.sub.2 O.sub.4, CuCr.sub.2 O.sub.4, and ZnFe.sub.2 O.sub.4 have been known for a long time. They are known for their often brilliant colors. Such substances have been used for the coloration of enamels and as a component of decorating colors. Although certain color pigments of the spinel type keep their color to a very high extent, when subjected to the effect of an aggressive glass flow, other spinels, particularly iron-containing spinels, show undesirable changes in color when used in enamels. Most spinels show the disadvantage of the so-called "drain effect." As a result of an interaction between the pigment and the glass flow, the pigment is dissolved to a lesser or greater extent; i.e., components of the pigment migrate into the surrounding glass flow and optionally color the surrounding glass. This "drain effect" inhibits the preparation of decorations having sharp contours or distinct color separation.
In order to increase the stability of color pigments during the fusing and/or firing of ceramic decorations, certain coloring agents have been included in transparent crystals of enamel-stable substances, as described in German Patent Application No. DE-AS 23 12 535, which is entirely incorporated herein by reference. Zirconium silicate is particularly suitable for use as a casing component which includes the colorizing pigment. The formation of zirconium silicate and the inclusion of the coloring particles generally take place in a solid phase reaction at a temperature between 750.degree. C. and 1400.degree. C., by using a silicon dioxide source and a zirconium dioxide source, preferably directly SiO.sub.2 and ZrO.sub.2, in the presence of a mineralizator. According to the process disclosed in DE-AS 23 12 535, it is not possible to include all conceivable coloring compounds during the formation of the zirconium silicate casing component in an amount required to exert the pigment effect. The coloring components and the pre-products thereof can often retard or prevent the formation of ZrSiO.sub.4 from SiO.sub.2 and ZrO.sub.2, or the coloring compounds may not even be included, and for this reason, the desired pigment properties can not be achieved.
In DE-AS 23 12 535, an experiment is described wherein a light blue inclusion pigment is obtained from SiO.sub.2, ZrO.sub.2 and Thenardblue, a spinel of the formula CoAl.sub.2 O.sub.4. It has been found that Cobalt-containing spinels of the formulae CoAl.sub.2 O.sub.4, CoFe.sub.2 O.sub.4, CoCr.sub.2 O.sub.4 can be included by the preparation of inclusion pigments into the zirconium silicate casing component either not at all, or only to a very limited extent. This relates to such cobaltcontaining spinels per se and to those formed by the in situ formation of spinels from CoO and Al.sub.2 O.sub.3, Fe.sub.2 O.sub.3 and Cr.sub.2 O.sub.3, respectively. When using the pigments thus obtained as decorating colors, significant traces and drain appear in the course of firing. The drain is believed to be due to the fact that cobalt is set free in the enamel.